Four states, including Vermont, Oregon, New Hampshire and Maine, earned an "A" on the report card as their preterm birth rates met the March of Dimes 9.6 percent goal.

"These results demonstrate that many premature births can be prevented with the right policies and bold leadership," March of Dimes President Dr. Jennifer L. Howse said in a prepared statement. "Our national progress in reducing premature births over the past five years shows that when infant health becomes a priority, babies benefit."

He said now, proven interventions must be implemented, and investment in new research must be accelerated to prevent preterm birth "so one day every baby will get a healthy start in life."

According to the National Center for Health Statistics, the U.S. preterm birth rate peaked in 2006 at 12.8, after rising for more than two decades. It dropped to 11.7 in 2011, which is the lowest in a decade.

Howse said all this improvement is a potential savings of about $3 billion in health care and economic costs to society. About 64,000 fewer babies were born preterm in 2010, when compared to 2006.

She said the improved birth rates were due to an expansion of successful programs and intervention, including actions by state health officials in 48 states, the District of Columbia and Puerto Rico.

The 2012 report card showed that 45 states, the District of Columbia, and Puerto Rico saw improvement in their preterm birth rates between 2009 and 2011.

The largest declines in premature birth took place among babies born at 34 to 36 weeks of pregnancy, but improvement was seen across the board. Every racial group benefited, and there were fewer preterm babies born at all stages of pregnancy.

The March of Dimes campaign "Healthy Babies are Worth the Wait" urges health care providers and patients not to schedule a delivery until at least 39 completed weeks of pregnancy.

The U.S. Department of Health and Human Services (HHS) "Strong Start" initiative is partnering with the March of Dimes to raise awareness about the importance of a full term pregnancy through paid advertising support and collaboration with hospitals to improve perinatal care.

The report card compared each star's preterm birth rate to the March of Dimes goal of lowering the rate to 9.6 percent by 2020.

It gauges states' progress towards lowering their preterm birth rates by tracking contributing factors. Seven states and the District of Columbia reduced the percentage of uninsured women of childbearing age, according to the report card.

March of Dimes also said that 43 sates, the District of Columbia and Puerto Rico lowered the late preterm birth rate (infants born between 34 and 36 weeks gestation).

About 15 million babies are born too soon each year and more than one million of those infants die as a result of their early births. The U.S. ranked 131 out of 184 countries, according to the May 2012 global report on premature birth.